The first generation of optical fiber systems in the public telephone network used proprietary architectures, equipment line codes, multiplexing formats, and maintenance procedures. This diversity complicated the task of the regional Bell operating companies (“RBOCs”) and the interexchange carriers (e.g., AT&T, Sprint, MCI, etc.) who needed to interface their equipment with these diverse systems.
To ease this task, Bellcore initiated an effort to establish a standard for connecting one optical fiber system to another. That standard is officially named the Synchronous Optical Network, but it is more commonly called “SONET.” The international version of the domestic SONET standard is officially named the Synchronous Digital Hierarchy, but it is more commonly called “SDH.”
Although differences exist between SONET and SDH, those differences are mostly in terminology. In most respects, the two standards are the same and, therefore, virtually all equipment that complies with either the SONET standard or the SDH standard also complies with the other. Therefore, for the purposes of this specification, the SONET standard and the SDH standard shall be considered interchangeable and the acronym/initialism “SONET/SDH” shall be defined as either the Synchronous Optical Network standard or the Synchronous Digital Hierarchy standard, or both.